Magnesium oxide particle aggregate

ABSTRACT

An object of the present invention is to provide a magnesium oxide particle aggregate having a controlled particle aggregation structure so that the solid phase-solid phase reaction between magnesium oxide and the SiO 2  film on the surface can be appropriately controlled. The object can be achieved by a magnesium oxide particle aggregate characterized in that a first inflection point diameter is 0.30×10 −6  m or less, an interparticle void volume is 1.40×10 −3  to 2.20×10 −3  m 3 /kg and a particle void volume is 0.55×10 −3  to 0.80×10 −3  m 3 /kg in a cumulative intrusion volume curve of the particle aggregate.

This application is a 371 of PCT/JP01/03702 filed Apr. 27, 2001.

FIELD OF THE INVENTION

The present invention relates to a magnesium oxide particle aggregatehaving a controlled particle aggregation structure. More particularly,the present invention relates to a magnesium oxide particle aggregateused as an annealing separator to form a forsterite film which impartsexcellent insulation properties and magnetic properties to agrain-oriented magnetic steel sheet.

BACKGROUND ART

Grain-oriented magnetic steel sheets used in transformers or generatorsare generally produced by a process in which silicon steel containingabout 3% of Si is hot-rolled, subsequently cold-rolled so as to have afinal sheet thickness, and then subjected to decarburization annealing(primary recrystallization annealing), followed by finishing annealing.In this process, for imparting insulation properties to a magnetic steelsheet, after the decarburization annealing and before the finalfinishing annealing, a slurry containing magnesium oxide is applied to asurface of the steel sheet and then dried, and wound into a coil form.Si contained in the silicon steel reacts with oxygen during thedecarburization annealing to form a SiO₂ film on the surface of thesteel sheet. SiO₂ in the film then reacts with magnesium oxide in theslurry during the finishing annealing to form a forsterite (Mg₂SiO₄)film having excellent insulation properties on the surface of the steelsheet. The forsterite film is considered to impart not only insulationproperties but also a tension to the surface thereof due to thedifference in the coefficient of thermal expansion between theforsterite film and the steel sheet, thus lowering core loss of thegrain-oriented magnetic steel sheet to improve the magnetic properties.

Therefore, the forsterite film plays an extremely important role in theproduction of grain-oriented magnetic steel sheets, and hence theproperties of magnesium oxide forming the forsterite film directlyaffect the magnetic properties thereof. For this reason, there haveconventionally been demands for magnesium oxide used as an annealingseparator to meet the requirements of excellent properties and resultantprecise control. In view of this, a number of inventions have been madewith respect to the magnesium oxide used as an annealing separator.

One example of such inventions is to add an additive to magnesium oxideor to control an impurity content thereof. For example, with respect tomagnesium oxide utilizing an additive, Japanese Patent Publication No.45322/1995 (process for producing a magnesium oxide composition)discloses a process for producing magnesium oxide, in which apredetermined amount of a boron compound is added to Mg (OH)₂ containingchlorine and then calcined under a predetermined steam partial pressure.

On the other hand, many inventions in respect of the activity determinedby the reaction rate between magnesium oxide particles and an acid,i.e., citric acid activity (CAA) are also disclosed. CAA is representedby a period of time required until a 0.4 N aqueous solution of citricacid at a predetermined temperature (for example, 22° C. or 30° C.)containing phenolphthalein as an indicator becomes neutral from a pointin time when a final reactive equivalent amount of magnesium oxide isadded to the solution and stirred. It is empirically known that CAA canbe used as an index for evaluation of the magnesium oxide used as anannealing separator for the grain-oriented magnetic steel sheet.

As an invention in respect of the distribution of CAA at a reactiveequivalent amount of magnesium oxide, Japanese Prov. Patent PublicationNo. 58331/1980 discloses an invention of magnesium oxide for anannealing separator having an activity adjusted so that the distributionof CAA is controlled in a narrow range for each final reaction rate of20%, 40%, 60% and 70%, respectively. In addition, Japanese Prov. PatentPublication Nos. 33138/1994 and 158558/1999 disclose an invention ofmagnesium oxide for an annealing separator, in which the activity of CAAof 40% or 80%, the particle diameter and the specific surface area arerestricted to respectively predetermined values. Further, Japanese Prov.Patent Publication No. 269555/1999 discloses an invention of anannealing separator for grain-oriented magnetic steel sheets, in whichCAA of 70%, the ratio of CAA of 70% to CAA of 40%, the particlediameter, the specific surface area and the like are restricted topredetermined values, respectively. In each of the above inventions, thehydration property and the reactivity of the magnesium oxide particlesare controlled.

The activity of magnesium oxide indicated by CAA is a yardstick for thereactivity in the solid phase-liquid phase reaction between magnesiumoxide and citric acid. In this solid phase-liquid phase reaction, thelarger the number of reactive sites in the solid phase, that is, thesmaller the particle diameter of magnesium oxide or the larger thespecific surface area thereof, the larger the surface free energy toincrease the activity.

However, in powder particles including magnesium oxide particlesproduced by various methods, oxide particles can be existed in the formof particle aggregate in which several powder particles are boundtogether and agglomerated in addition to the case existed in the form ofindividual particle. In the particle aggregate caused by agglomerationor aggregation, the CAA measured is not a value reflecting the structureof the particle aggregate and therefore cannot precisely represent thereactivity of an annealing separator.

Further, CAA merely simulates empirically the reactivity in the solidphase-solid phase reaction, which actually proceeds between SiO₂ andmagnesium oxide on the surface of the magnetic steel sheet, using thesolid phase-liquid phase reaction between magnesium oxide and citricacid. Differing from the solid phase-liquid phase reaction, in theforsterite formation reaction which is a solid phase-solid phasereaction, the particle aggregation structure of magnesium oxide, forexample, the number of contact points between the SiO₂ film and themagnesium oxide particles, is presumed to remarkably affect thereactivity. Specifically, even when the magnesium oxide particles haveactive surfaces, a small number of contact points derived from theparticle aggregation structure cause the reaction to proceedunsatisfactorily. On the other hand, even when magnesium oxide particleshave inactive surfaces, an increased number of contact points canadvance the reaction satisfactorily.

As mentioned above, CAA used as an index of the properties of anannealing separator for the magnetic steel sheet is a yardstick forevaluation of the reactivity of magnesium oxide only under givenconditions. It is considered that CAA dose not necessarily preciselyevaluate the solid phase-solid phase reaction which actually proceeds onthe surface of the magnetic steel sheet. Therefore, in magnesium oxidehaving a poor activity evaluated by CAA, there is a possibility thatmagnesium oxide having a particle aggregation structure suitable for anannealing separator can be found by using a method of controlling thesolid phase-solid phase reaction taking into consideration theaggregation structure of powder particles.

In view of the above, an object of the present invention is to provide amagnesium oxide particle aggregate having a controlled particleaggregation structure so that the solid phase-solid phase reactionbetween magnesium oxide and the SiO₂ film on the surface can beappropriately controlled. An object of the present invention is furtherto provide an annealing separator for a grain-oriented magnetic steelsheet, using the magnesium oxide particle aggregate of the presentinvention, and to provide a grain-oriented magnetic steel sheetobtainable by a treatment using the annealing separator of the presentinvention.

DISCLOSURE OF THE INVENTION

The present inventors have conducted extensive and intensive studies tosolve the above-mentioned problems and thus completed the presentinvention. Specifically, the present invention is a magnesium oxideparticle aggregate characterized in that, in a cumulative intrusionvolume curve of particles, a first inflection point diameter is0.30×10⁻⁶ m or less, an interparticle void volume is 1.40×10⁻³ to2.20×10⁻³ m³/kg and a particle void volume is 0.55×10⁻³ to 0.80×10⁻³m³/kg.

In addition, the present invention is an annealing separator for agrain-oriented magnetic steel sheet, using the magnesium oxide particleaggregate having the above particle aggregation structure.

Further, the present invention is a grain-oriented magnetic steel sheetobtainable by a treatment using the above annealing separator.

In the present invention, a cumulative intrusion volume curve ofparticles means a curve which shows a relationship between a porediameter and a cumulative pore volume determined from a poredistribution measurement by mercury porosimetry. FIG. 1 shows twocumulative intrusion volume curves of different types of magnesium oxideparticle aggregates having different particle aggregation structures. Afirst inflection point is an inflection point at the largest porediameter among inflection points at which the cumulative intrusionvolume curve suddenly rises. It is indicated by a solid circle in thefigure. A first inflection point diameter means a pore diameter at thefirst inflection point. An interparticle void volume means a cumulativepore volume at the first inflection point. A particle void volume meansa volume obtained by subtracting the cumulative pore volume at the firstinflection point from the total pore volume.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is graphs showing examples of cumulative intrusion volume curvesof particle aggregates comprised mainly of magnesium oxide, determinedfrom a pore distribution measurement by mercury porosimetry.

FIG. 2 is a graph showing a relationship between a forsterite formationrate, a first inflection point diameter and a particle void volume withrespect to various MgO particle aggregates.

FIG. 3 is a graph showing a relationship between a forsterite formationrate, a first inflection point diameter and an interparticle void volumewith respect to various MgO particle aggregates.

FIG. 4 is a graph showing a relationship between a particle void volumeand an interparticle void volume with respect to the particle aggregatehaving a first inflection point diameter of 0.30×10⁻⁶ m or less.

FIG. 5 is graphs showing temperature and time conditions suitable forcontrolling the reaction so that the first inflection point diameterbecomes 0.30×10⁻⁶ m or less when magnesium hydroxide is prepared byreacting an aqueous solution of magnesium chloride with calciumhydroxide.

BEST MODE FOR CARRYING OUT THE INVENTION

The present inventors have made studies on the solid phase-solid phasereaction between magnesium oxide and silica (SiO₂), which reactionproceeds on a surface of a grain-oriented magnetic steel sheet, from theviewpoint of the particle aggregation structure. As a result, they havefound that a first inflection point diameter, a particle void volume andan interparticle void volume, in a cumulative intrusion volume curvedetermined by a pore distribution measurement using mercury porosimetry,can be used as indices for precisely indicating the structure of amagnesium oxide particle aggregate. Based on the above finding, theseindices are controlled so as to fall in respective appropriate ranges tocreate a magnesium oxide particle aggregate which can appropriatelycontrol a number of contact points in the solid phase reaction so thatthe magnesium oxide particle aggregate is surely reacted with the SiO₂film formed on the surface of the grain-oriented magnetic steel sheet toform forsterite. The magnesium oxide particle aggregate having acontrolled particle aggregation structure exhibits an appropriatereaction rate of the forsterite formation and forms a large amount offorsterite, and further can form a forsterite film having good adhesion.

The pore distribution measurement by mercury porosimetry for obtainingindices indicating the particle aggregation structure was conducted bythe following method. The method for the pore distribution measurementby mercury porosimetry is well known as an analysis method for obtainingdata of a pore distribution of powder and thus data of a particleaggregation structure.

As a mercury porosimeter, AutoPore 9410, manufactured by MicromeriticsGbmH, was used. A measurement cell for powder sample having a cellcapacity of 5×10⁻⁶ m³ and a stem capacity of 0.38×10⁻⁶ m³ was used. Asample to be measured was preliminarily passed through a 330 meshstandard sieve (JIS-R8801-87) and then precisely weighed in the range offrom 0.10×10⁻³ to 0.13×10⁻³ kg, and placed in the measurement cell. Thecell was set in the porosimeter, and then the inside of the cell wasmaintained in a reduced pressure of 50 μHg (6.67 Pa) or less for 20minutes. Next, mercury was charged into the measurement cell until thepressure in the cell became 1.5 Pisa (10,342 Pa). Then, the mercury waspressed under a pressure in the range of from 2 Pisa (13,790 Pa) to60,000 Pisa (413.7 MPa) to measure a pore distribution. As the mercuryfor the measurement, a special grade mercury reagent having a purity of99.5 mass % or higher was used, and the density of the mercury used was13.5335×10³ kg/m³.

The data obtained from the pore distribution measurement by mercuryporosimetry was plotted on a graph in which a pore diameter determinedfrom the mercury pressing pressure was taken as an abscissa and acumulative pore volume as an ordinate, so that a cumulative intrusionvolume curve shown in FIG. 1 was obtained. The mercury pressing pressureis converted to a pore diameter using the following formula(I)(Washburn's equation).

 D=−(1/P)×4γ×cos φ  (I)

wherein D: pore diameter (m);

P: pressure (Pa);

γ: surface tension of mercury {485 dyne/cm (0.485 Pa^(·)m)}; and

φ: contact angle of mercury (130°=2.26893 rad).

A first inflection point diameter, a particle void volume and aninterparticle void volume were individually determined from thecumulative intrusion volume curve as follows.

In the cumulative intrusion volume curve of FIG. 1, a cumulative porevolume on the ordinate indicates a cumulative value of a pore volume ina particle aggregate per unit weight determined from larger poressuccessively. An inflection point is a point at which the cumulativeintrusion volume curve suddenly rises. The number of inflection point isnot necessarily one, and there is a case where a plurality of inflectionpoints are present depending on a sample to be measured, as can be seenin curve B in FIG. 1. The inflection point at the largest pore diameteris thus taken as a first inflection point. A first inflection pointdiameter is the pore diameter at the first inflection point. Aninterparticle void volume is a void volume between the aggregateparticles. It is represented by the cumulative pore volume at the firstinflection point. A particle void volume is a void volume which ispresent in the particles and smaller than the diameter of the aggregateparticles. It is represented by the volume value obtained by subtractingthe cumulative pore volume at the first inflection point from the totalpore volume.

The relationship between each of the first inflection, the interparticlevoid volume and the particle void volume point in a cumulative intrusionvolume curve and the particle aggregation structure of the particleaggregate is presumed as follows.

Powder particle generally constitute an aggregate particle byaggregation of a plurality of the smallest unit particles (primaryparticles), each of which is presumed to be a single particle. Theaggregation structures of powder include a simple particle structuresuch that primary particles merely agglomerate and a complicatedparticle structure such that secondary aggregates formed by aggregationof primary particles further agglomerate to form a larger tertiaryaggregation structure. Particle aggregation may be occurred by variousreasons, for example, aggregation caused by surface charges of particlesdispersed in a liquid, deposition of the ingredients dissolved duringdrying, electrostatic charge in a dry state, a physical stress duringgrinding or grain boundary growth during calcination. Thus, powderfrequently has a characteristic ordered particle structure depending ontypes or conditions of production processes and qualities of rawmaterials.

When taking such a particle structure into consideration, the firstinflection point diameter indicates a size of the largest orderedaggregation structure among these ordered aggregation structures. Theparticle void volume is a volume of a pore smaller than the aggregateparticles, and it is an index of the density of the aggregate particles.The interparticle void volume is a void volume between the aggregateparticles in a state such that the aggregate particles are in contactwith one another. The void volume between the aggregate particles incontact with one another increases as the unevenness of the surface ofthe aggregate particles becomes larger. Therefore, the interparticlevoid volume can be used as a material property value indirectlyindicating the form of the aggregate particles. Thus, each of the firstinflection point diameter indicating a size of the ordered aggregateparticle structure, the particle void volume indicating the density ofthe aggregate particles and the interparticle void volume indirectlyindicating the form of the aggregate particles is a suitable materialfactor for indicating the complicated aggregate particle structure.Therefore, in the powder comprised of a homogeneous material, forexample, magnesium oxide, the particle aggregation structure can becontrolled so as to achieve predetermined values by appropriatelyselecting the production conditions for the powder.

When pressing mercury into a particle aggregate having the aboveparticle structure, mercury first penetrates into the voids between theparticles. In this instance, as the mercury pressing pressure increases,that is, the pore diameter determined from the mercury pressing pressuredecreases, the cumulative pore volume increases with a substantiallyconstant gradient. After all voids between the particles are filled withmercury, mercury starts penetrating into the voids in the particles. Agreat number of voids having the same size are present in the particles,and the sum of the voids in the particles are large. Therefore, when thepenetration of mercury is changed from the voids between the particlesto the voids in the particles, the cumulative pore volume drasticallyincreases even as the mercury pressing pressure slightly increases. Inother words, the pore diameter at which the cumulative pore volumedrastically increases is presumed to correspond to the first inflectionpoint diameter which is the maximum value for the aggregate unitstructure in the particle aggregate.

The particle void volume is a volume determined by subtracting thecumulative pore volume at the first inflection point from the total porevolume. The total pore volume is deemed a cumulative pore volume at apore diameter of 0.003×10⁻⁶ m. This is because the particle structure ischanged due to the pressing pressure in the pore distributionmeasurement by mercury porosimetry, and therefore, the measurement errorcan be minimized by using the cumulative pore volume at the maximummercury pressing pressure as a total pore volume.

Next, magnesium oxide particle aggregates having different particleaggregation structures in which each of the first inflection pointdiameter, the particle void volume and the interparticle void volume, ina cumulative intrusion volume curve, are individually different wereprepared to examine the reaction rate of the solid phase reactionbetween the individual magnesium oxide particle aggregates and silica.

The magnesium oxide particle aggregate was prepared using magnesiumchloride as a raw material, and calcium hydroxide was added to anaqueous solution of magnesium chloride to effect a reaction, thusforming magnesium hydroxide. Then, the magnesium hydroxide was subjectedto filtration by means of a filter press, and washed with water anddried, and then calcined using a rotary kiln to form magnesium oxide.The calcined magnesium oxide was ground.

The preparation of the magnesium oxide particle aggregate is not limitedto the above-mentioned method. A number of methods for preparation canbe employed, for example, a method in which an alkaline aqueous solutionsuch as an aqueous solution of calcium hydroxide, sodium hydroxide orpotassium hydroxide is reacted with a magnesium chloride-containingaqueous solution such as bittern, brackish water or sea water, to obtainmagnesium hydroxide, and then the magnesium hydroxide is calcined toobtain a magnesium oxide particle aggregate; a method in which magnesiteis calcined to obtain a magnesium oxide particle aggregate; a method inwhich a magnesium oxide particle aggregate is directly obtained from amagnesium chloride-containing aqueous solution by an amalgamationmethod; and a method in which magnesium oxide obtained by theabove-mentioned method is subjected to hydration to form magnesiumhydroxide, followed by calcination, to obtain a magnesium oxide particleaggregate. By appropriately selecting conditions for the individualsteps, the particle structure can be controlled.

The first inflection point diameter and the particle void volume of themagnesium oxide particle aggregate were adjusted by controlling theparticle structure of magnesium hydroxide which is a precursor ofmagnesium oxide under the below-described conditions. On the other hand,the interparticle void volume was adjusted by controlling thebelow-described grinding conditions. In the present invention, forelucidating the effect of the magnesium oxide particle aggregationstructure on the reaction rate of the forsterite solid phase reaction,the temperature for calcination of magnesium hydroxide by means of arotary kiln was set to 800 to 1,000° C. so that CAA of 40% at a finalreaction rate at 22° C. was within the range of from 120 to 140.

On the other hand, in the solid phase-solid phase reaction betweenmagnesium oxide and silica (SiO₂), these were directly reacted with eachother to form forsterite. Specifically, the magnesium oxide particleaggregate prepared by the above-mentioned method and amorphous silicawas mixed in a molar ratio of 2:1 to form a mixture. The mixture wasthen shaped under a pressure of 50 MPa to obtain a shaped article havinga diameter of 15×10⁻³ m and a height of 15×10⁻³ m. The shaped articlewas then calcined in a nitrogen gas atmosphere at 1,200° C. for 4 hours.An X-ray diffraction analysis was conducted to quantitatively determinea forsterite formation rate in the sintered product obtained by theprocess.

Table 1 shows index values for the particle aggregation structure andforsterite formation rate for 22 samples measured. FIG. 2 shows arelationship between a forsterite formation rate, the first inflectionpoint diameter and a particle void volume. FIG. 3 shows a relationshipbetween a forsterite formation rate, a first inflection point diameterand an interparticle void volume. FIG. 4 further shows a relationshipbetween a particle void volume and an interparticle void volume withrespect to the particle aggregate having the first inflection pointdiameter of 0.30×10⁻⁶ m or less. In the figures, the forsteriteformation rate is indicated by classifying into three levels, i.e., 90%or more, 80 to less than 90%, and less than 80%.

TABLE 1 Forsterite First inflection Interparticle void formation Sam-point diameter Particle porosity volume rate ple 10⁻⁶ m 10⁻³ m³ kg⁻¹10⁻³ m³ kg⁻¹ % A 0.17 0.77 2.16 91.3 B 0.18 0.88 1.31 86.2 C 0.17 0.832.44 84.7 D 0.23 0.65 1.68 91.9 E 0.23 0.66 1.35 89.7 F 0.24 0.65 2.3386.5 G 0.21 0.85 1.81 89.5 H 0.22 0.58 1.43 90.5 I 0.25 0.56 2.12 90.1 J0.22 0.52 1.59 86.8 K 0.27 0.74 1.46 91.8 L 0.26 0.49 1.31 81.2 M 0.270.53 2.27 82.1 N 0.45 0.71 1.51 87.2 O 0.43 0.72 1.28 84.9 P 0.44 0.722.29 83.6 Q 0.43 0.91 1.62 81.0 R 0.42 0.48 1.45 78.8 S 0.59 0.52 2.2577.0 T 0.61 0.48 1.34 75.5 U 0.88 0.84 1.27 77.7 V 0.90 0.85 2.32 75.6

For stably achieving a forsterite formation rate of 90% or more, as canbe seen from FIG. 2, it is necessary that the first inflection pointdiameter be 0.30×10⁻⁶ m or less and the particle void volume be withinthe range of from 0.55×10⁻³ to 0.80×10⁻³ m³/kg, and, as can be seen fromFIG. 3, it is necessary that the first inflection point diameter be0.30×10⁻⁶ m or less and the interparticle void volume be within therange of from 1.40×10⁻³ to 2.20×10⁻³ m³/kg. Here, the forsteriteformation rate of 90% or more was taken as a reference value. When theforsterite formation rate can satisfy such a reference value, thereactivity in the solid phase reaction between the magnesium oxideparticle aggregate and the film comprised mainly of silica formed on thesurface of the steel sheet is high, and the film having excellentadhesion properties can be formed in the forsterite formation reaction.

From the results shown in FIGS. 2 and 3, since the smaller the firstinflection point diameter, the higher the reactivity of the particleaggregate, and therefore, it is an essential feature that the firstinflection point diameter is as small as possible. However, by meetingonly the requirement that the first inflection point diameter be0.30×10⁻⁶ m or less, forsterite cannot be stably obtained in a formationrate of 90% or more. Specifically, in solid phase-solid phase reactionof the forsterite formation, the number of contact points between themagnesium oxide particles and the silica particles or the silica filmdetermines the rate of the solid phase reaction, and hence optimizationof the number of the above contact points is needed. The numbers of thecontact points depend on the interparticle void volume and the particlevoid volume of the particle aggregate. As shown in FIG. 4, the particleaggregate must have a particle aggregation structure which satisfies therequirement that the first inflection point diameter be 0.30×10⁻⁶ m orless, the interparticle void volume be within the range of from1.40×10⁻³ to 2.20×10⁻³ m³/kg and the particle void volume be within therange of from 0.55×10⁻³ to 0.80×10⁻³ m³/kg.

Next, a magnesium oxide particle aggregate having a first inflectionpoint diameter, an interparticle void volume and a particle void volume,each being in the above-mentioned appropriate range, can be prepared asfollows. It is noted that the preparation method described below ismerely an example, and a magnesium oxide particle aggregate having theparticle aggregation structure defined in the present invention can beprepared by other methods.

The first inflection point diameter and the particle void volume of themagnesium oxide particle aggregate are adjusted by controlling theparticle structure of magnesium hydroxide which is a precursor ofmagnesium oxide. Specifically, a calcium hydroxide slurry is added to amagnesium chloride solution so that the resultant magnesium hydroxideconcentration becomes a predetermined value, and the resultant mixtureis stirred to effect a reaction at a predetermined temperature for apredetermined time, and then, the reaction mixture is subjected tofiltration by means of a filter press, washed with water and dried toform magnesium hydroxide.

For adjusting the first inflection point diameter to be 0.30×10⁻⁶ m orless, as shown in FIG. 5, magnesium hydroxide is formed by a reactionunder conditions such that the reaction temperature (T, ° C.) and thereaction time (t, hr) satisfy the relationship represented by thefollowing formula (II).

3,230 epx(−0.1476^(·) T)≦t≦217,000 epx(−0.0855^(·) T)  (II)

When the reaction time exceeds 217,000 epx(−0.0855^(·)T), the firstinflection point diameter becomes more than 0.30×10⁻⁶ m, result in toolarge aggregate particles. On the other hand, the reaction time is lessthan 3,230 epx(−0.1476^(·)T), the reaction of magnesium hydroxideformation does not proceeds satisfactorily. It is more preferred thatthe reaction temperature (T, ° C.) and the reaction time (t, hr) satisfythe relationship represented by the following formula (III).

 2,350 epx(−0.103^(·) T)≦t≦20,000 epx(−0.0896^(·) T)  (III)

For adjusting the particle void volume to be within the range of from0.55×10⁻³ to 0.80×10⁻³ m³/kg, the ratio between the magnesium chloridesolution and the calcium hydroxide slurry mixed is adjusted so that themagnesium hydroxide concentration after the reaction becomes 0.2 to 4.5mol/kg, preferably 0.5 to 3 mol/kg. When the magnesium hydroxideconcentration after the reaction is less than 0.2 mol/kg, a particleaggregate having such a low density that the particle void volume ismore than 0.80×10⁻³ m³/kg is disadvantageously formed. On the otherhand, when the magnesium hydroxide concentration after the reactionexceeds 4.5 mol/kg, a particle aggregate having too high a density suchthat the particle void volume is less than 0.55×10⁻³ m³/kg isdisadvantageously formed.

In the reaction of forming magnesium hydroxide, a flocculant can beadded for promoting the aggregation reaction, and a flocculationpreventing agent can be added for preventing the aggregation reactionfrom proceeding to an excess extent. Examples of flocculants includealuminum sulfate, polyaluminum chloride, iron sulfate andpolyacrylamide, and preferred are polyaluminum chloride and anionicpolyacrylamide. The flocculant can be added in an amount of 1 to 1,000ppm, preferably 5 to 500 ppm, more preferably 10 to 100 ppm, based onthe total mass of the magnesium chloride solution and the calciumhydroxide slurry. It is not preferred to add a flocculant in an excessamount since a particle aggregate having too high a density such thatthe particle void volume is less than 0.55×10⁻³ m³/kg isdisadvantageously formed.

On the other hand, as a flocculation preventing agent, sodium silicate,sodium polyphosphate, sodium hexametaphosphate, a nonionic surfactant oran anionic surfactant can be added, and preferred are sodium silicate,sodium hexametaphosphate and nonionic surfactants. The flocculationpreventing agent can be added in an amount of 1 to 1,000 ppm, preferably5 to 500 ppm, more preferably 10 to 100 ppm, based on the total mass ofthe magnesium chloride solution and the calcium hydroxide slurry. It isnot preferred to add a flocculation preventing agent in an excess amountsince a particle aggregate having such a low density that the particlevoid volume is more than 0.88×10⁻³ m³/kg is disadvantageously formed.

The stirring was conducted at a stirring rate of 350 to 450 rpm. Thestirring does not largely affect the particle structure, but theinterparticle void volume can be increased by stirring at a high speedand at a high shear rate by means of, for example, a homogenizer duringthe reaction or can be lowered by almost no stirring.

Next, the thus formed magnesium hydroxide precursor is calcined by meansof a rotary kiln to form a magnesium oxide particle aggregate. In thiscase, the calcination temperature may be 800 to 1,000° C., preferably850 to 950° C. The calcination time may be 0.2 to 4 hours, preferably0.5 to 2 hours.

Further, the obtained magnesium oxide particle aggregate is ground usinga hammer mill grinder at a power of 5.5 kW having a classifier. Forobtaining a particle aggregate having the interparticle void volume inthe range of from 1.40×10⁻³ to 2.20×10⁻³ m³/kg, the hammer rotationalfrequency may be 2,800 to 4,200 rpm, especially preferably 3,200 to3,800 rpm. When the hammer rotational frequency is less than 2,800 rpm,the resultant particle structure has too high a density and a desiredinterparticle void volume cannot be obtained. On the other hand, therotational frequency of the classifier is preferably 2,200 to 4,800 rpm,more preferably 2,800 to 4,200 rpm. When the classifier rotationalfrequency exceeds 5,000 rpm, the resultant particle structure has a lowdensity and a desired interparticle void volume cannot be obtained.

As a grinder, a hammer mill grinder, a high-speed rotating mill grinder,a jet mill grinder, a roller mill grinder or a ball mill grinder can beused. The optimal conditions of the grinder for obtaining theinterparticle void volume which falls in the range defined in thepresent invention vary depending on the system and ability (power) ofthe grinder used, but too strong grinding increases the interparticlevoid volume and too weak grinding lowers the interparticle void volume.In the jet mill grinder in which the impact energy applied duringgrinding is large, the impact energy may lower the particle void volume,and therefore the operation of the grinder of this type is needed to becontrolled under conditions suitable for the apparatus. Further, aclassifier is not necessarily used, but the use of a classifier makes itpossible to control the grinding conditions more flexibly.

Next, using the thus obtained magnesium oxide, an annealing separatorfor a grain-oriented magnetic steel sheet and a grain-oriented magneticsteel sheet are produced-as follows.

A grain-oriented magnetic steel sheet is produced as follows. A siliconsteel slab having an Si content of 2.5 to 4.5% is hot-rolled, thenpickled with an acid, followed by cold rolling or coldrolling-intermediate annealing-cold rolling so that the resultant sheethas a predetermined thickness. Then, the cold-rolled coil is subjectedto recrystallization annealing, which also effects decarburization, in awet hydrogen gas atmosphere at 700 to 900° C. to form an oxide filmcomprised mainly of silica (SiO₂) on the surface of the steel sheet. Anaqueous slurry obtained by uniformly dispersing in water the magnesiumoxide particle aggregate having the particle aggregation structure ofthe present invention prepared by the above method is continuouslyapplied onto the resultant steel sheet using a roll coater or a spray,and dried at about 300° C. The thus treated steel sheet coil issubjected to final finishing annealing, for example, at 1,200° C. for 20hours to form forsterite (Mg₂SiO₄) on the surface of the steel sheet.The forsterite imparts a tension to the surface thereof along with theinsulating film to improve core loss of the grain-oriented magneticsteel sheet.

As described in, for example, Japanese Prov. Patent Publication No.101059/1994, for facilitating the forsterite film formation, a knownreaction accelerator, inhibitor auxiliary or tension-impartinginsulating film additive can be added to the annealing separator.

EXAMPLES

Next, the present invention will be described in more detail withreference to the following Examples.

Example 1

Magnecite ore was calcined by means of a rotary kiln at 850° C. for 1hour and ground by means of a cage mill grinder to form magnesium oxide.For controlling the particle structure, the magnesium oxide was added towater so that the magnesium hydroxide concentration after reactionbecame 3 mol/kg, and the resultant mixture was stirred at 400 rpm toeffect a reaction at 85 to 95° C. for 2 hours, thus forming magnesiumhydroxide as a precursor. The magnesium hydroxide was subjected tofiltration by means of a filter press, and washed with water and dried,and then the resultant magnesium hydroxide was calcined by means of arotary kiln at 900° C. for 1 hour so that CAA of 40% at a final reactionrate at 22° C. fell in the range of from 120 to 140 to obtain a calcinedmagnesium oxide particle aggregate. Further, the calcined aggregate wasground using a hammer-type grinder at a hammer rotational frequency of3,500 rpm and at a classifier rotational frequency of 3,000 rpm toprepare a magnesium oxide particle aggregate having a predeterminedparticle aggregation structure.

Example 2

A calcium hydroxide slurry was added to bittern so that the magnesiumhydroxide concentration after reaction became 2 mol/kg, and theresultant mixture was stirred at 600 rpm to effect a reaction at 80° C.for 2 hours. Then, the reaction mixture was subjected to filtration bymeans of a filter press, and washed with water and dried, and theresultant magnesium hydroxide was calcined by means of a rotary kiln at890° C. for 1 hour so that CAA of 40% at a final reaction rate at 22° C.fell in the range of from 120 to 140 to obtain a calcined magnesiumoxide particle aggregate. Further, the calcined aggregate was groundusing a grinder at a hammer rotational frequency of 3,500 rpm and at aclassifier rotational frequency of 4,000 rpm to prepare a magnesiumoxide particle aggregate having a predetermined particle aggregationstructure.

Example 3

A calcium hydroxide slurry was added to magnesium chloride so that themagnesium hydroxide concentration after reaction became 1 mol/kg, andpolyaluminum chloride was added thereto as a flocculant in an amount of10 ppm, and the resultant mixture was stirred at 400 rpm to effect areaction at 60° C. for 20 hours. Then, the reaction mixture wassubjected to filtration by means of a filter press, and washed withwater and dried, and the resultant magnesium hydroxide was calcined bymeans of a rotary kiln at 900° C. for 1 hour so that CAA of 40% at afinal reaction rate at 22° C. fell in the range of from 120 to 140 toobtain a calcined magnesium oxide particle aggregate. Further, thecalcined aggregate was ground using a grinder at a hammer rotationalfrequency of 3,200 rpm and at a classifier rotational frequency of 4,000rpm to prepare a magnesium oxide particle aggregate having apredetermined particle aggregation structure.

Comparative Example 1

Bittern and calcium hydroxide were reacted with each other to formmagnesium hydroxide, and the magnesium hydroxide was calcined by meansof a rotary kiln at 950° C., and then placed in water and heated againto effect a reaction at 80° C. for 2 hours, followed by filtration anddrying. The resultant magnesium hydroxide was calcined by means of amuffle kiln at an internal temperature of 1,200° C. to prepare magnesiumoxide particles. The thus prepared particles are not controlled withrespect to the particle aggregation structure as conducted in thepresent invention, but they are magnesium oxide for an annealingseparator currently for a high performance grain-oriented magnetic steelsheet.

Comparative Example 2

Bittern and calcium hydroxide were reacted with each other at 40° C. for10 hours to form magnesium hydroxide, and then the magnesium hydroxidewas calcined by means of a rotary kiln at 1,050° C. to prepare magnesiumoxide particles. The thus prepared particles are not controlled withrespect to the particle aggregation structure as conducted in thepresent invention, but they are magnesium oxide for an annealingseparator used for general magnetic steel sheets.

Comparative Example 3

Calcium hydroxide was added to sea water so that the magnesium hydroxideconcentration after reaction became 0.05 mol/kg to effect a reaction at50° C. for 20 hours, thus forming magnesium hydroxide. 5 Hours beforecompletion of the reaction, anionic polyacrylamide was added in anamount of 200 ppm, and the reaction mixture after completion of thereaction was subjected to filtration by means of a filter press anddried. Then, the resultant magnesium hydroxide was calcined by means ofa rotary kiln at 950° C. to prepare magnesium oxide particles. The thusprepared particles are not controlled with respect to the particleaggregation structure as conducted in the present invention, and theyare magnesium oxide used in an application other than the annealingseparator.

Table 2 shows the measurement results for particle aggregationstructures of the particles or particle aggregates in Examples andComparative Examples. As can be seen from the table, in each of Examples1 to 3 in which the particle aggregate was produced while controllingthe particle aggregation structures, the first inflection point diameteris very small and both the interparticle void volume and the particlevoid volume fall in the respective ranges defined in the presentinvention, and the reactivity in the solid phase reaction is essentiallyexcellent. On the other hand, in each of Comparative Examples in whichthe particle aggregation structure was not controlled, the firstinflection point diameter is larger than the upper limit of the range ofthe pore diameter defined in the present invention. Especially inComparative Example 3 in which the particles are used in an applicationother than the annealing separator, the first inflection point diameteris very large. Further, the particles in Comparative Example 1 have aparticle aggregation structure such that the interparticle void volumeis lower than the lower limit of the range defined in the presentinvention, the particles in Comparative Example 2 have a particleaggregation structure such that both the interparticle void volume andthe particle void volume are larger than the respective upper limits ofthe ranges defined in the present invention, and the particles inComparative Example 3 have a particle aggregation structure such thatthe interparticle void volume is lower than the lower limit of the rangedefined in the present invention.

TABLE 2 First inflection Interparticle void point diameter Particleporosity volume 10⁻⁶ m 10⁻³ m³ kg⁻¹ 10⁻³ m³ kg⁻¹ Example 1 0.12 0.611.99 Example 2 0.28 0.69 1.45 Example 3 0.20 0.59 1.85 Comp. ex. 1 0.350.71 1.16 Comp. ex. 2 0.35 0.84 2.63 Comp. ex. 3 1.17 0.62 0.52

With respect to the above magnesium oxide particle aggregates or powderparticles, the behavior of formation of a forsterite film was examined.It is presumed that the formation of forsterite proceeds according tothe solid phase reaction: 2MgO+SiO₂→Mg₂SiO₄. Therefore, a magnesiumoxide powder and amorphous SiO₂ were mixed in a molar ratio of 2:1 toform a mixture, and the mixture was shaped under a pressure of 50 MPa toobtain a shaped article having a diameter of 15×10⁻³ m and a height of15×10⁻³ m. Then, the shaped article was calcined in a nitrogen gasatmosphere at 1,200° C. for 4 hours. This calcination temperaturecorresponds to the temperature of the finishing annealing in which SiO₂is reacted with a slurry containing magnesium oxide on thegrain-oriented magnetic steel sheet. An X-ray diffraction analysis wasconducted to quantitatively determine an Mg₂SiO₄ formation rate for theobtained sintered product. The results are shown in Table 3.

TABLE 3 Mg₂SiO₄ formation rate (mass %) Example 1 90.6 Example 2 93.2Example 3 91.8 Comp. ex. 1 89.6 Comp. ex. 2 77.5 Comp. ex. 3 63.4

As can be seen from Table 3, in each of Examples 1 to 3, the forsteriteformation rate exceeds 90%, and thus excellent effect of the magnesiumoxide particle aggregate having a particle aggregation structurecontrolled with respect to the first inflection point diameter and theparticle void volume as well as the interparticle void volume isachieved. In addition, it is apparent that the forsterite formation ratein each of Examples 1 to 3 is higher than that of the magnesium oxide inComparative Example 1, which is currently used as an annealing separatorfor a high-grade grain-oriented magnetic steel sheet. Further, theforsterite formation rate of the magnesium oxide in Comparative Example2 used as a general annealing separator and that of the magnesium oxidein Comparative Example 3 used in an application other than the annealingseparator are very small.

Next, magnesium oxide was applied to a magnetic steel sheet to examinethe properties of a forsterite film. A silicon steel slab for agrain-oriented magnetic steel sheet, which slab comprises C: 0.058%; Si:2.8%; Mn: 0.06%; Al: 0.026%; S: 0.024%; N: 0.0050% (in terms of % bymass); and the balance of unavoidable impurities and Fe, was hot-rolled,pickled with an acid and cold-rolled by known methods to Obtain thefinal sheet thickness of 0.23 mm, and followed by decarburizationannealing in a wet atmosphere comprised of 25% of nitrogen gas and 75%of hydrogen gas.

The magnesium oxide particle aggregates of the present invention and themagnesium oxide particles in Comparative Examples, each in a slurryform, were individually applied to the above steel sheet so that thedried coating weight became 12 g/m², and dried and then, subjected tofinal finishing annealing at 1,200° C. for 20 hours. The forsteritefilms formed on the steel sheets are shown in Table 4.

TABLE 4 State of glass film formed Evaluation Example 1 Uniform andthick ⊚ Example 2 Uniform and thick ⊚ Example 3 Uniform and thick ⊚Comp. ex. 1 Uniform and slightly thin ◯ Comp. ex. 2 Nonuniform andslightly thin Δ Comp. ex. 3 Nonuniform and very thin X

As can be seen from Table 4, the forsterite films formed from theparticle aggregates in Examples 1 to 3 are those having a uniform andsatisfactory thickness. Particularly, it is apparent that the particleaggregates in Examples 1 to 3 form a forsterite film having anappropriate thickness and are excellent annealing separators, ascompared with the magnesium oxide particles in Comparative Example 1which are currently used as an annealing separator for a high-gradegrain-oriented magnetic steel sheets.

INDUSTRIAL APPLICABILITY

As mentioned above, in the present invention, there can be providedmagnesium oxide having a particle aggregation structure which canadvantageously form forsterite. In addition, the magnesium oxideparticle aggregate of the present invention exhibits high forsteriteformation rate, as compared with the magnesium oxide currently used asan annealing separator for a grain-oriented magnetic steel sheet.Therefore, the grain-oriented magnetic steel sheet obtainable by atreatment using the magnesium oxide of the present invention hassatisfactory magnetic properties as a grain-oriented magnetic steelsheet. Further, the technical concept of the present invention can beapplied not only to the annealing separator but also to other solidphase reactions, for example, ceramic synthesis.

What is claimed is:
 1. A magnesium oxide particle aggregate having afirst inflection point diameter of 0.30×10⁻⁶ m or less, an interparticlevoid volume of 1.40×10⁻³ to 2.20×10−3 m³/kg and a particle void volumeof 0.55×10⁻³ to 0.80×10⁻³ m³/kg in a cumulative intrusion volume curveof said particle aggregate.